Multi-ring hydraulic seal for irregular bore surfaces

ABSTRACT

The sealing of irregular bore surfaces against the leakage of high pressure hydraulic fluid is accomplished by a multi-ring unit including an elastomer ring supported between a pair of teflon rings which also provide respective support for a rearward and a forward oversize leather ring, a steel cam ring adjacent the forward face of the forward leather ring, and an aluminum alloy wedge ring cooperating with the cam ring to provide an unsupported area therebetween. When the sealing unit is mechanically compressed after insertion thereof into the bore, the elastomer and the wedge rings are each partially extruded into circumferential obturating contact with the irregular bore surface to effect adequate sealing thereof until the sealing rings as well as the support rings are fully deformed into the final sealing configuration.

United States Patent [1 1 Brown Uct. 23, 1973 MULTI-RKNG HYDRAULIC SEAL FOR IRREGULAR BORE SURFACES [75] Inventor: Bruce B. Brown, Latham, NY.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: Sept. 1, 1972 [21] Appl. No.: 285,943

52' 11.5.01. 277/125 [51] Int. Cl F16j 15/24 [58] Field of Search 277/102, 117, 125

[56] References Cited i V UNITED STATES PATENTS 228,341 6/1880 Gould 277/125 332,793 12/1885 Dickson 277/125 1,694,822 1211928 Kennedy. 277/117 2,249,155 7/1941 Meddick 277/117 2,292,543 8/1942 Patterson.. 277/125 3,068,017 12/1962 Prosser 277/117 3,171,492 3/1965 Cochran 277/117 3,197,215 7/1965 Hodge 277/125 3,467,394 9/1969 Bryant 277/125 Primary ExaminerSamuel B. Rothberg Assistant ExaminerRobert 1. Smith Attorney-Harry M. Saragovitz et a1.

[57] ABSTRACT The sealing of irregular bore surfaces against the leakage of high pressure hydraulic fluid is accomplished by a multi-ring unit including an elastomer ring supported between a pair of teflon rings which also provide respective support for a rearward and a forward oversize leather ring, a steel cam ring adjacent the forward face of the forward leather ring, and an aluminum alloy .wedge ring cooperating with the cam ring to provide an unsupported area therebetween. When the sealing unit is mechanically compressed after insertion thereof into the bore, the elastomer and the wedge rings are each partially extruded into circumferential obturating contact with the irregular bore surface to effect adequate sealing thereof until the sealing rings as well as the support rings are fully deformed into the final sealing configuration.

10 Claims, 7 Drawing Figures Patented Oct. 23, 1973 3,767,215

2 Sheets-Sheet 1 MULTli-RTNG ll-HYDRAJULHC @EAL IFOllt llHtRlEGlUlLAllt BGRIE SURFACES BACKGROUND OF THE INVENTION This invention relates to the sealing of high pressure fluids and is more particularly directed to a multi-ring seal for use in a hydraulic pressure technique which simulates the stresses produced within the bore of a gun tube during the firing of a round of ammunition therein.

in the past, the establishment of the safe fatigue life for the type of gun tube employed in large caliber artillery weapons has generally been accomplished by the actual firing of one or more representative tubes to the point of actual failure. Since a firing program of such magnitude is extremely expensive and time-consuming, considerable effort has been directed to developing a technique capable of simulating the stresses produced in the gun tube during actual firing. Relatively recent successes have demonstrated that it is possible to achieve the desired simulation by cycling hydraulic fluid through the gun tube in repetitive pulses equivalent in pressure and duration to those encountered in the tiring of successive rounds ofammunition.

However, in view of the difficulties encountered in sealing the relatively irregular rifled interior of a tired gun tube against the leakage of high pressure hydraulic fluid, the simulation of firing stresses has heretofore been limited to sections cut from tubes which had been subjected to sufficient firing to fully develop the cracks which are ultimately responsible for the fatigue failure of the tube. in order to positively seal the tube section during the firing simulation, it has been found necessary to machine finely finished pockets at each end thereof to properly seat the conventional seals. While effective insofar as sealing is concerned, the machining of these pockets is both costly and time-consuming and, in conjunction with the required sectioning of the gun tube, frequently resulted in the loss of important portions of partially worn rifting. Of even greater concern has been the considerable difficulty experienced in correlating the growth rate of the cracks propagated during the simulated firing with those developed under actual firing conditions. Obviously, the firing of even one gun tube to the point of failure introduces the same costly and time-consuming procedure the simulated firing is specifically intended to eliminate. Moreover, in

view of the statistical nature of gun tube fatigue life and the inherent variability of the materials from which the tubes are fabricated, a fairly extensive testing program is required to provide a meaningful comparison of the crack growth rate under actual and simulated conditions.

It has therefore been suggested that the necessary correlation could be achieved more accurately and economically by alternating selected amounts of actual and simulated firings on the same gun tube. Such procedure would not only permit a direct comparison of the rate of growth of the fatigue cracks under the two different firing modes involved but would also greatly reduce the amount of actual firing required. However, the successful conduct of this desirable technique is dependent on the positive sealing against hydraulic leakage of the relatively rough and unpredictably irregular surfaces of the alternate lands and grooves present in the rifled interior of the gun tubes.

SUMMARY OF THE HNVENTION It is therefore an object of this invention to provide an improved high pressure seal which will fully obturate the annular clearance between a tube and a cylindrical seal carrier inserted therein despite the considerable roughness and irregularity of the bore surface of the tube.

A further object of the present invention is to provide an improved seal, as aforesaid, wherein a plurality of relatively deformable rings of different material and configuration are alternated with supporting rings of a more rigid construction.

Another object of this invention lies in the provision of a multi-ring seal, as aforesaid, which can be mechanically deformed to provide an initial sealing action against the leakage of hydraulic fluid during the first few cycles of hydraulic actuation required to complete the full sealing deformation thereof.

Still another object of this invention is to provide a multi-ring seal, as aforesaid, wherein the more rigid rings support the deformable rings in a manner which minimizes the extrusion thereof in the direction parallel to the surface to be sealed.

It is a still further object of this invention to provide a multi-ring seal, as aforesaid, which can be readily and completely removed from a rifled gun tube to avoid any interference with the subsequent re-firing thereof.

' It has been found that the foregoing objects can be achieved by a seal assembly carried on a mandrel to be inserted into the bore of a gun tube. The assembly includes an elastomer ring of modified O-ring configuration supported between a pair of teflon rings, an oversized leather ring adjacent each of the teflon rings, a steel ring adjacent to one of the leather rings and provided with a frustoconical end face on the side furthest from the leather ring, and a wedge ring of aluminum alloy having an end face of greater slope than the angular face of the steel ring to form an unsupported area therebetween. The opposite face of the wedge ring is formed at a slight angle to the vertical to provide partial contact thereof with the end face of the adjacent closure member utilized to retain the seal assembly in place. Thus, when the closure member is mechanically advanced to compress the seal assembly, the Wedge ring is subjected to an overturning force which causes a portion of the exterior periphery thereof to extrude into initial sealing contact with the bottom of the rifling grooves and the sides of the adjacent lands in the gun tube. At the same time, the elastomer ring is bulged outwardly to provide a similar initial obturating contact with the rifling grooves and lands. As a result, the elastomer, leather, and Wedge rings provide adequate sealing contact during the first few cycles of hydraulic pressure required to complete the full deformation thereof into obturating contact with the rifled interior bore of the gun tube. While the sealing rings are permitted to readily deform in the direction perpendicular to the gun tube surface to be sealed, the teflon rings, the steel ramp ring, and the steel end closures each provide the support necessary to minimize the extrusion of the sealing rings in the direction parallel to the rifled surface to be sealed.

in the case of the wedge ring such limitation is additionally enforced by the work hardening produced 7' during the extensive deformation imparted thereto by the high hydraulic pressure.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of the invention, as well as other objects and advantages thereof, will be readily apparent from consideration of the following specification relating to the annexed drawings wherein:

FIG. 1 is a vertical section through the breech end portion of a rifled gun tube equipped with an arrangement for cycling hydraulic fluid therethrough in a manner simulating the stresses encountered during actual firing and shows the means for sealing the opposite ends of the stressed portion of the tube;

FIG. 2 is an enlarged sectional view of the forward sealing means of FIG. 1 with the mandrel portion shown in full and the configuration and the materials of the individual components thereof shown prior to the application of any pressure thereon;

FIG. 3 is a perspective view of the wedge ring shown in FIG. 2 and is partially cut away to show the crosssectional configuration thereof;

FIG. 4 is a fragmentary sectional view of the sealing means of FIG. 2 which is still further enlarged to show the position of the individual components following the application of the mechanical pressure required to provide the desired initial sealing deformation thereof;

FIG. 5 is a fragmentary section taken along line 5-5 in FIG. 4 to show the relationship of the partially deformed wedge ring to the rifling grooves and lands in the gun tube;

FIG. 6 is a view equivalent to that of FIG. 4 but showing the position and configuration of the individual components of the seal assembly following the completion of the deformation required to positively seal the gun tube portion undergoing the hydraulic cycling designed to simulate firing stresses; and

FIG. 7 is a fragmentary view of FIG. 6 showing the extent of the sealing deformation imparted to the various rings in the seal where the surface to be sealed is relatively rough and irregular in contour.

DESCRIPTION OF A PREFERRED EMBODIMENT The seal of the present invention is particularly successful in the type of rough and irregular surface provided by the rifled interior of a large caliber gun which has been tired to a considerable extent. As best shown in FIG. 1, such gun generally includes a breech ring 12 threaded to the breech end of a gun tube 14 and arranged to seat the slidable breechblock 16 therefor. The bore of the gun tube 14 is customarily provided with alternate helical lands l8 and grooves 20. In order to simulate the stresses normally encountered in firing a plurality of live rounds in the gun, hydraulic fluid is cycles through a portion of tube 14 in a manner which will provide a repeated rise and fall in pressure comparable to actual firing pressure in intensity. This is accomplished by high pressure tubing 22 passing through breech ring 12 and breechblock 16 into the rear end of a cylindrical mandrel 24 which extends forwardly into the bore of gun tube 14. Surrounding the rear end of mandrel 24 is a conventional O-ring unit 26 arranged to seal the breech end of gun tube 14 against the rearward leakage of the hydraulic fluid. Mandrel 24 is also provided with a suitable forwardly extending passage 28 designed to direct the hydraulic fluid into the annular clearance between the exterior periphery of mandrel 24 and the rifled bore surface of gun tube 14. The forward portion of mandrel 24 is reduced in diameter,

as indicated at 30, and terminates in a threaded end 32 engageable with an internally threaded cylindrical rifling pilot 34 of hollow configuration preferably fabricated of a relatively hard bearing metal such as beryllium copper. Forward movement of pilot 34 relative to mandrel 24 is prevented by a circular plate 36 secured to the front end of mandrel 24 as by bolts 38. The exterior circumference of pilot 34 is formed with a series of helically oriented spaced lugs 40 arranged to slidably engage in rifling grooves 20 as mandrel 24 is guided into the breech end of gun tube 14. However, when mandrel 24 is rotated, any accompanying rotation of pilot 34 is prevented by rifling lands l8 and consequently, mandrel 24 will be threadably advanced relative to pilot 34 for a purpose to be hereinafter shown.

Reduced diameter portion 30 of mandrel 24 terminates in an arcuate rear end wall 42 and cooperates with the rifled bore surface of gun tube 14 to provide an annular recess for seating a series of sealing elements of which the rearwardmost is a steel closure ring 44 having an inner periphery conforming to the arcuate curvature of end wall 42 and an outer periphery of slightly lesser diameter than that across rifling lands l8. Adjacent to closure ring 44 is an oversized leather ring 46 arranged to fully conform to the irregular contour of the rifling in gun tube 14 as mandrel 24 is inserted therein. A support ring 48 of teflon is seated next to leather ring 46 and is formed with a concave forward face 50 conforming to the arcuate rear end face 51 of an adjacent ring 52 of elastomeric material. The body of ring 52 is axially tapered to form a forward annular face 54 of smaller diameter than the adjacent face of a support ring 56 of teflon. A second oversize leather ring 46 is positioned between teflon support ring 56 and a steel cam ring 58 having a frustoconical front face 60.

Between ring 58 and the closure end 61 of rifling pilot 34 is a wedge ring 62 of aluminum alloy having a rear face 64 of sufficient slope to contact cam ring 58 only along the inner portion of front face so as to provide an unsupported area therebetween. The front face 66 of wedge ring 62 is also sloped relative to the vertical and is additionally provided with a rounded edge 68 blending into the inner periphery thereof. The outer periphery of wedge ring 62 is forwardly sloped away from the bore surface of gun tube 14. Except for leather rings 46, the major diameters of the other rings are each slightly less than the diametrical distance between rifling lands 18 in order to prevent scoring or other undesirable mutilation thereof as mandrel 24 is being inserted into the bore of gun tube 14.

In order to seal the annular clearance between mandrel 24 and the rifled bore surface of gun tube 14 during the first few hydraulic pressure cycles prior to the completion of the deformation incurred by rings 46, 52, and 62, sufficient mechanical pressure is imparted to the entire assembly to effect the deformation required to insure a preliminary seal. This is accomplished by rotating mandrel 24, preferably from the rear end thereof, to advance forwardly relative to rifling pilot 34 thereby compressing the resilient rings of the assembly to partially deform as shown in FIG. 4. Elastomer ring 52 is bulged into an essentially oval configuration which enters into each of rifling grooves 20 to provide a sealing contact along a circumferential area in gun tube 14. At the same time, teflon support ring 48 is extruded in the radial direction to partially enter into rifling grooves to a progressively increasing depth. Support ring 56 is also partially extruded into rifling grooves 20 but to a relatively constant depth. In addition, wedge ring 62 is partially rotated by the reduction in the distance between ring 56 and closure end 61 of rifling pilot 34 so that the outer rear portion of ring 62 is forced into circumferential sealing contact with the floor of rifling grooves 20. As best shown in FIG. 4, the major portion of wedge ring face 64 is still clear of angular face 60 of ring 58 while the major portion of the exterior periphery thereof is similarly clear of the floor of rifling grooves 20.

However, once a few cycles of hydraulic pressure are applied, all the rings except closure ring 44; are forced forwardly as shown in FIG 6 to complete the required sealing deformation. The exterior peripheries of leather rings 46 and teflon support rings 4-0 and 56 are each radially deformed into full sealing contact with rifling lands l8 and grooves 20 with only a slight amount of extrusion in the direction parallel to the bore surface. Leather rings 46 are considerably more compressible than teflon rings 48 and 56 and therefore absorb a greater share of the existing hydraulic pressure, thereby reducing the amount of extrusion imparted to rings 48 and 56 in the direction parallel to the bore surface. While the bulging of elastomer ring 52 has been increased, support rings 48 and 56 serve to limit the axial extrusion of ring 52 along rifling lands 1% and grooves 20. The absence of such support has heretofore been the factor most responsible for the rapid deterioration of O-ring type seals under the high pressures here involved, generally in the vicinity of 60,000 psi. The aluminum wedge ring 62 has also been completely deformed in a radial direction into full sealing contact with rifling grooves 20 with considerably lesser axial extrusion into the clearance between lugs 40 on rifling pilot 34 and the rifling lands and grooves in guntube M. It is believed that such desirable result is primarily due to the unique configurations of camring 58 and wedge ring 62 and the relationship therebet ween. As the unsupported area of rear face 64 is progressively reduced, the force tending to deform wedge ring 62 is correspondingly diminished thereby minimizing further deformation in the axial direction once the exterior surface thereof is in full sealing contact with the rifling in gun tube 14. The controlled extrusion of wedge ring 62 may also be partially due to the work hardening of the aluminum alloy during the deformation thereof.

Thus, there is here provided an extremely successful sealing arrangement for obturating the interior of a rifled gun tube against thelealtage of hydraulic fluid under pressures ranging between 40,000 and 70,000 psi. Adequate sealing is provided throughout the fatigue life of the gun tube even through the rifling therein may be partially worn and eroded thereby substantially increasing the irregularity of the surface to be sealed in addition to the existing stepped configuration of the rifling lands and grooves. Furthermore, despite the extensive deformation of the sealing rings into the gun tube rifling, the entire assembly may be readily removed from gun tube M merely by retracting mandrel 24 relative to rifling pilot 34 to reduce the compression on the rings and thereafter breaking the engagement thereof with the rifling to permit mandrel 24 to be retracted from gun tube 114i. Such procedure is accomplished without any adverse effect on the interior of the gun tube which may then be subjected to additional actual firing.

The foregoing disclosure and description of the invention is illustrative only. Various changes may be made within the scope of the appended claims without departing from the spirit of the invention.

1 claim:

1. A multi-ring seal for obturating a relatively rough and irregular bore surface against the leakage of hydraulic fluid under high pressure comprising,

a pair of leather rings of slightly larger diameter than the bore surface to be sealed,

an elastomer ring,

a pair of support rings of molded plastic straddling said elastomer ring and disposed between said leather rings,

a cam ring of steel in adjacent contact with one of said leather rings, and

a wedge ring of aluminum alloy cooperating with said cam ring to form an unsupported area therebetween for increasing the extrusion imparted thereto under hydraulic pressure to a greater extent in the direction at right angles to the surface to be sealed than in the direction parallel thereto.

2. The seal defined in claim 1 wherein said elastomer ring is formed with a convex face tapering to an opposite annular face of smaller diameter whereby the extrusion thereof in response to the application of hydraulic pressure is greater in the direction perpendicular to the bore surface to be sealed than in the direction parallel thereto.

3. The seal defined in claim 1 wherein the face of said wedge ring adjacent to said cam ring is of substantially frustoconical configuration in order to provide the unsupported area which permits limited sealing orientation thereof prior to the application of the hydraulic pressure.

4. A multi-ring seal for obturating a relatively rough and irregular bore surface within a tube against the leakage of hydraulic fluid under high pressure compris ing,

a pair of spaced leather rings of sufficient diameter to fully engage the bore surface to be sealed upon initial insertion thereof into the tube,

a pair of support rings of molded plastic of lesser diameter than said leather rings and respectively in contact with adjacent surfaces thereof,

an elastomer ring having a portion of its diameter equal to said plastic support rings and seated therebetween,

a steel cam ring adjacent one ofsaid leather rings,

an aluminum alloy wedge ring having an end face adjacent to said cam ring to form an unsupported area therebetween, and

means for mechanically compressing each of said rings in the seal whereby said elastomer ring and said wedge ring are extruded into partial obturating contact with the bore surface to effect positive sealing thereof during the period in which the hydraulic pressure is extruding the seal into final maximum sealing obturation.

d. The seal defined in claim 4 wherein one face of said elastomer ring is supported along a greater surface area than the opposite face to minimize the extrusion thereof in the direction parallel to the bore surface to be sealed.

6. The seal defined in claim 4 wherein said means for mechanically compressing said rings comprises,

a cylinder having a reduced diameter portion for mounting said rings to permit the insertion thereof into the tube,

a closure member threaded on said cylinder in adjacent contact with said wedge ring,

means for preventing rotation of said closure member relative to the tube, and

a closure ring between the reduced diameter portion of said ring mounting cylinder and the other of said leather rings whereby rotation of said cylinder relative to said closure member advances said closure ring to compress the remaining rings in the seal.

7. A multi-ring seal mounted on a mandrel adapted to be inserted into the breech end of a rifled gun tube for circumferentially obturating the lands and grooves therein against the forward leakage of hydraulic fluid being cycled therethrough to simulate the stresses ordinarily encountered in the firing of a round therein, comprising,

a pair of spaced leather rings of sufficient diameter to fill the rifling grooves in the gun tube during the insertion of the mandrel therein,

a pair of support rings of teflon of lesser diameter than said leather rings and respectively in contact with adjacent surfaces thereof,

an elastomer ring having a portion of its diameter equal to said teflon rings and seated therebetween,

a steel cam ring having a frustoconical front face and a rear face in contact with the forwardmost of said leather rings,

a wedge ring of aluminum alloy having a rear face cooperating with said front face of said cam ring to provide an unsupported area therebetween,

a pilot rotatably secured to the forward end of the mandrel and engageable in the rifling grooves in the gun tube, and

means for longitudinally advancing said mandrel relative to said pilot to reduce the space occupied by the seal whereby said elastomer and wedge rings are partially extruded into circumferential sealing engagement in the bore of the gun tube.

8. The seal defined in claim 7 wherein said pilot comprises an internally threaded hollow cylinder having a plurality of helically inclined lugs spaced about the exterior periphery thereof to permit insertion of the mandrel into the gun tube and yet prevent rotation of the cylinder in response to rotation of the mandrel.

9. The seal defined in claim 7 wherein said wedge ring includes a front face inclined at a relatively small angle to the rear face of said pilot whereby the partial extrusion of said wedge ring is attained without eliminating the unsupported area provided thereby in cooperation with said cam ring.

10. The seal defined in claim 7 wherein said elastomer ring comprises an arcuate rear face tapering to an annular front face of lesser diameter and wherein said teflon ring in contact with said rear face of said elastomer ring is correspondingly contoured to provide full support thereto against axial extrusion parallel to the rifled surface to be sealed. 

1. A multi-ring seal for obturating a relatively rough and irregular bore surface against the leakage of hydraulic fluid under high pressure comprising, a pair of leather rings of slightly larger diameter than the bore surface to be sealed, an elastomer ring, a pair of support rings of molded plastic straddling said elastomer ring and disposed between said leather rings, a cam ring of steel in adjacent contact with one of said leather rings, and a wedge ring of aluminum alloy cooperating with said cam ring to form an unsupported area therebetween for increasing the extrusion imparted thereto under hydraulic pressure to a greater extent in the direction at right angles to the surface to be sealed than in the direction parallel thereto.
 2. The seal defined in claim 1 wherein said elasTomer ring is formed with a convex face tapering to an opposite annular face of smaller diameter whereby the extrusion thereof in response to the application of hydraulic pressure is greater in the direction perpendicular to the bore surface to be sealed than in the direction parallel thereto.
 3. The seal defined in claim 1 wherein the face of said wedge ring adjacent to said cam ring is of substantially frustoconical configuration in order to provide the unsupported area which permits limited sealing orientation thereof prior to the application of the hydraulic pressure.
 4. A multi-ring seal for obturating a relatively rough and irregular bore surface within a tube against the leakage of hydraulic fluid under high pressure comprising, a pair of spaced leather rings of sufficient diameter to fully engage the bore surface to be sealed upon initial insertion thereof into the tube, a pair of support rings of molded plastic of lesser diameter than said leather rings and respectively in contact with adjacent surfaces thereof, an elastomer ring having a portion of its diameter equal to said plastic support rings and seated therebetween, a steel cam ring adjacent one of said leather rings, an aluminum alloy wedge ring having an end face adjacent to said cam ring to form an unsupported area therebetween, and means for mechanically compressing each of said rings in the seal whereby said elastomer ring and said wedge ring are extruded into partial obturating contact with the bore surface to effect positive sealing thereof during the period in which the hydraulic pressure is extruding the seal into final maximum sealing obturation.
 5. The seal defined in claim 4 wherein one face of said elastomer ring is supported along a greater surface area than the opposite face to minimize the extrusion thereof in the direction parallel to the bore surface to be sealed.
 6. The seal defined in claim 4 wherein said means for mechanically compressing said rings comprises, a cylinder having a reduced diameter portion for mounting said rings to permit the insertion thereof into the tube, a closure member threaded on said cylinder in adjacent contact with said wedge ring, means for preventing rotation of said closure member relative to the tube, and a closure ring between the reduced diameter portion of said ring mounting cylinder and the other of said leather rings whereby rotation of said cylinder relative to said closure member advances said closure ring to compress the remaining rings in the seal.
 7. A multi-ring seal mounted on a mandrel adapted to be inserted into the breech end of a rifled gun tube for circumferentially obturating the lands and grooves therein against the forward leakage of hydraulic fluid being cycled therethrough to simulate the stresses ordinarily encountered in the firing of a round therein, comprising, a pair of spaced leather rings of sufficient diameter to fill the rifling grooves in the gun tube during the insertion of the mandrel therein, a pair of support rings of teflon of lesser diameter than said leather rings and respectively in contact with adjacent surfaces thereof, an elastomer ring having a portion of its diameter equal to said teflon rings and seated therebetween, a steel cam ring having a frustoconical front face and a rear face in contact with the forwardmost of said leather rings, a wedge ring of aluminum alloy having a rear face cooperating with said front face of said cam ring to provide an unsupported area therebetween, a pilot rotatably secured to the forward end of the mandrel and engageable in the rifling grooves in the gun tube, and means for longitudinally advancing said mandrel relative to said pilot to reduce the space occupied by the seal whereby said elastomer and wedge rings are partially extruded into circumferential sealing engagement in the bore of the gun tube.
 8. The seal defined in claim 7 wherein said pilot comprises an internally Threaded hollow cylinder having a plurality of helically inclined lugs spaced about the exterior periphery thereof to permit insertion of the mandrel into the gun tube and yet prevent rotation of the cylinder in response to rotation of the mandrel.
 9. The seal defined in claim 7 wherein said wedge ring includes a front face inclined at a relatively small angle to the rear face of said pilot whereby the partial extrusion of said wedge ring is attained without eliminating the unsupported area provided thereby in cooperation with said cam ring.
 10. The seal defined in claim 7 wherein said elastomer ring comprises an arcuate rear face tapering to an annular front face of lesser diameter and wherein said teflon ring in contact with said rear face of said elastomer ring is correspondingly contoured to provide full support thereto against axial extrusion parallel to the rifled surface to be sealed. 